feat: added messageStylePrefix variable

[Techn1x]

Enable the ability to not use the generic bootstrap or foundation class names, by adding flashTypePrefixmessageStylePrefix which provides a way for the user to specify the flash type class name prefix.

Backwards compatibility is maintained with messageStyle, but if flashTypePrefixmessageStylePrefix is set it will take priority.

{{#each flashMessages.queue as |flash|}}
  <FlashMessage @flash={{this.flash}} @messageStylePrefix="special-alert-" />
{{/each}}

would produce an element like

<div class="flash-message special-alert-success" > ... </div>

Rationale

I've been running a custom flash-message component for this addon for a while now, because;

• the flash-message component in this addon was a bit old
• the classes alert and alert-box are very overused and poorly scoped in our 15-20 frontend apps... we really needed to avoid using these class names, and this addon didn't provide a way to customize that (without resorting to a custom component)

I noticed that with v2.0.0 the component has been updated to be tagless, render modifiers etc so I thought I would contribute some of my workings back to the project, with the aim of being able to use the component in this addon again!

[Techn1x]

btw! I am participating in hacktoberfest this year so I would much appreciate it if this pull request could have the hacktoberfest-accepted label added to it, or if the repo could have hacktoberfest label/topic :100:

[Techn1x]

Any interest in merging this @sbatson5 ? It's a requirement for me to not need to use a custom flash component

[Techn1x]

Thanks @sbatson5 those are all great suggestions, implemented them all

EDIT: except for the dev warning